Thermoelectric elements first came into use for AC voltage and current measurements in the late 1920's and were first used for AC-DC transfer measurements in the 1940's at the U.S. Bureau of Standards, now NIST. The AC to DC transfer technique has since become widely used in thermal converters and thermal transfer standard measuring instruments for highest level precision measurement and calibration world-wide.
The use of a series resistor to extend the range of a low-voltage voltmeter has been widely practiced for many years. Such a resistor is sometimes termed a “range” resistor because it extends the range of the meter, or a “multiplier” resistor because it in effect multiplies the instrument range usually by a selected integer value, or an “attentuator” because it attenuates the high voltage under measurement to the low voltage of the meter by forming a voltage divider in conjunction with the resistance of the metering instrument.
Multi-meters in common use may contain several or many such range resistors built into the instrument and made selectable by a switch. However for high accuracy laboratory measurements the switch is eliminated as a possible source of error, and instead the range resistor is packaged as a separate module to be connected to the instrument by high quality virtually lossless connectors.
The meter resistance is a key parameter that may be formed at least in part by one or more precision resistors. For purposes of AC to DC comparison techniques for measuring RMS or heating effect of AC voltages especially those with non sinusoidal waveform, this “meter” or load resistance may be formed at least in part by a special thermocouple or other thermoelectric load, which may present a load resistance that is much lower than the resistance of the measuring circuit or meter.
Since a thermocouple is sensitive only to the mean square heating effect of the applied voltage or current, its output response is virtually independent of the waveform of the applied input voltage or its frequency. The device is therefore very useful for AC-DC transfer measurements because it responds nearly equally to DC and to the RMS value of AC current.
A thermal element with a single thermocouple can typically develop a 10 millivolt output with an input current of 5 to 10 milliamperes full scale, depending on the device, but is limited to not less than one half of the full scale input to maintain good resolution, due to the square law transfer function of the heater element.
The thermocouple heating element operates at a low voltage, typically 0.5 to 1.0 volt, necessitating the use of a range resistor in series to measure higher voltages in the range of tens to thousands of volts.